Interpretive Summary: Soil fumigants are highly volatile pesticides and widely used to kill soil-borne pests and pathogens before planting crops. Research has shown that a significant fumigant fraction (about 20-90%) can be emitted into the atmosphere after soil fumigation, which increases health risks. To mitigate atmospheric emissions, many new application methods have been developed that include restricting fumigant movement, accelerating fumigant degradation, injecting fumigants further from the soil surface, and enhancing downward fumigant transport using drip-line application. While experimental testing methods are useful for determining the effect of application methodology on fumigant emissions from soil, they are also relatively complex, time-consuming, and expensive; and often cannot be suitably replicated. Process-based mathematical models provide a relatively simple and cost-effective alternative to expensive laboratory and field experiments to simulate pesticide transport and fate; however, relatively few modeling efforts to simulate fumigant transport and fate have been published. The objective of this study was to predict methyl iodide (MeI) transport and fate under various emission reduction strategies and simultaneously predict the effects of the emission reduction strategies on soil pest control efficacy using simulation models. The results show that use of a virtually impermeable film led to the lowest MeI emissions and that integrated methods, such as deep + HDPE and Reagent + irrigation, are also recommended. This research would be of interest to scientists, regulators, cooperative extension, growers and film manufacturers.

Technical Abstract:
Various methods have been developed to reduce atmospheric emissions from the agricultural use of highly volatile pesticides and mitigate their adverse environmental effects. The effectiveness of various methods on emissions reduction and pest control was assessed using simulation model in this study. Firstly, validation comparison between the simulated and laboratory measured results was made for methyl iodide (MeI) period-averaged emission rates and cumulative emissions under four emission reduction treatments, including 1) control, 2) organic matter addition (HOM), 3) virtually impermeable film (VIF), and 4) surface irrigation (Irrigation). Then the model was extended to simulate a broader range of emission reduction strategies for MeI, including 5) high density polyethylene (HDPE), 6) deep injection (Deep), 7) HDPE + Deep, 8) reactive surface (Reagent), 9) Reagent + Irrigation, and 10) Reagent + HDPE. Finally, the survivability of three types of soil-borne pests (citrus nematodes [Tylenchulus semipenetrans], barnyard seeds [Echinochloa crus-galli], fungi [Fusarium oxysporum]) was also predicted based on the dose-response curves from Luo at al., (2011). Overall, the trend and shape of the measured emission fluxes as well as total emission were reasonably reproduced by the model for treatments 1 through 4. Based on the model, the ranking of effectiveness in total emission reduction was VIF (82.4 %)> Reagent + HDPE (73.2 %) > Reagent+ Irrigation (43.0%) > Reagent (23.5%) > Deep + HDPE (19.3%) > HOM (17.6%)> Deep (13.0%) > Irrigation (11.9%)> HDPE (5.8%). However, the order for pest control efficacy was different. Generally, VIF had the highest pest control efficacy, followed by deep + HDPE, irrigation, Reagent + irrigation, HDPE, Deep, Reagent+ HDPE, Reagent, and HOM. Therefore, VIF is the optimized method disregarding the cost of the film. Otherwise, the integrated methods such as deep + HDPE and Reagent + irrigation, are recommended.